Microsoft Office 365 offers access to all office apps, whereas Office 2021 offers access to only core office apps(Word, Excel, PowerPoint, and Outlook.)
Microsoft Office 365 : Monthly/yearly payments. Office 2021: One-time payment.
Microsoft Office 365 : Free Upgrades. Office 2021: Paid Upgrades.
Office 2021 has no customer support.
Office 2021 has no support for older windows.
Microsoft Office 365 : Users can share account with upto 6 people. Office 2021: Only single licence.
Microsoft Office 365 : Some features require Internet. Office 2021: Totally offline.